In blasting operations, various devices are used to transmit a blast signal from a remote initiation location to explosives in a borehole. These devices include transmission lines, delays, downlines, and detonators that are arranged in such a manner as to detonate explosives in a desired sequence and pattern. In nonelectric systems, delay elements are interposed along the transmission lines and in the boreholes to establish a blast pattern. The transmission lines carry the blast signal from an initiator to downlines or to surface delay devices. Downlines transmit the signal from transmission lines or surface delay elements to the explosives in the boreholes. The downlines may be attached to delay devices in the borehole and/or to instantaneous blasting caps in the borehole. The explosives within a borehole may also be decked, i.e., loaded in explosive sections that detonate at different times. The use of delay devices to detonate the boreholes in a predesigned pattern also helps to reduce noise and vibration incident to blasting operations which is important in light of governmental regulations and complaints from nearby residents. Due to these advantages, the industry has made wider use of delay devices.
A disadvantage in using delays has been the increased potential for malfunctions in the blast caused by the breaking of transmission lines and downlines prior to being activated. Such breaks may be caused by a line being severed by shifting or falling rock that is set in motion by the first portion of the blasting sequence. Malfunctions may also be due to defective transmission lines or delay devices. As a result of these malfunctions, one or more boreholes may fail to detonate because they do not receive a firing signal. The industry requires reliable detonation because of the hazards involved when a charge of explosives is not detonated.
The usual means of assuring total detonation of a pattern of explosives is to provide a redundant backup circuit. In the past, this was difficult and very costly to do and still achieve the desired delays of the pattern. The present invention provides a simple and economical means of providing a backup signal to insure reliable detonation and a system that maintains the desired sequence in a blasting pattern. The invention includes an apparatus that is easy to construct and simple to use.